Problem with versions and remote systems

These are our products in system A:(dbsstt12) $ isql -VINFORMIX-SQL Version 7.20.UD1 $ i4gl -VINFORMIX-4GL Version 7.20.UD1 $ onstat -VInformix Dynamic Server Version 7.30.UC2 Software Serial Number AAC#J410101 ----------------------------------- In system B: (dbsstt46) $isql -V INFORMIX-SQL Version 4.12.UE1 $ i4gl -V INFORMIX-4GL Version 4.10.UC2 and RSAM Version 5.02.UC7 ----------------------------------- And our BIG problem: 1. The local system is A. We ask for this select: select * from pagano a where a.dni_ci="11111111H" and not exists (select b.dni_ci from sucesion@dbsstt46:pagano b where b.dni_ci=a.dni_ci) and it works correctly. Instead of this, we use 'not in', as follows: select * from pagano a where a.dni_ci="11111111H" and a.dni_ci not in (select b.dni_ci from sucesion@dbsstt46:pagano b where b.dni_ci=a.dni_ci) and the system falls down, with this message: 25587 Network receive failed 2. The local system is B. We ask for this select: select * from pagano a where a.dni_ci="11111111H" and not exists (select b.dni_ci from sucesion@dbsstt12:pagano b where b.dni_ci=a.dni_ci) and it works correctly. Instead of this, we use 'not in', as follows: select * from pagano a where a.dni_ci="11111111H" and a.dni_ci not in (select b.dni_ci from sucesion@dbsstt46:pagano b where b.dni_ci=a.dni_ci) and also works well. What is the problem with 'NOT IN'?